G-protein-coupled receptors (GPCRs) are the largest class of cell-surface receptors (more than 1000 genes are present in the human genome). They can be activated by a diverse array of stimuli, e.g. hormones, peptides, amino acids, photons of light, and these receptors play a large role in the central nervous system and in the periphery. GPCRs are proteins with 7 transmembrane domains with highly conserved domains. It was estimated that in the year 2000 half of all modern drugs and almost one-quarter of the top 200 best-selling drugs are directed against or modulate GPCR targets (approximately 30 in total). However, due to their architecture of 7 membrane-spanning helices and their strong tendency to aggregate, it's a very challenging target class.
GPCRs are a well-known class of receptors. Reference is for example made to the following reviews: Surgand et al., Proteins 62:509-538 (2006); Vassilatis et al., Proc Natl Acad Sci USA 100:4903-4908 (2003) and Pierce et al., Nat Rev Mol Cell Biol 3:639-650 (2002); as well as to for example: George et al., Nat Rev Drug Discov 1:808-820 (2002); Kenakin, Trends Pharmacol Sci 25:186-192 (2002); Rios et al., Pharmacol Ther 92:71-87 (2001); Jacoby et al., ChemMedChem 2006, 1, 760-782; and Schlyer and Horuk, Drug Discovery Today, 11, 11/12. June 2006, 481; and also for example to Rosenkilde, Oncogene (2001), 20, 1582-1593 and Sadee et al., AAPS PharmSci 2001; 3; 1-16; as well as to the further references cited therein.
G-protein-coupled receptors (GPCRs) are the largest class of cell-surface receptors (more than 1000 genes are present in the human genome). They can be activated by a diverse array of stimuli, e.g. hormones, peptides, amino acids, photons of light, and these receptors play a large role in the central nervous system and in the periphery. GPCRs are proteins with 7 transmembrane domains with highly conserved domains.
As half of all known drugs work through G-protein coupled receptors, it is commercially very attractive to select Nanobodies against this protein family. It was estimated that in the year 2000 half of all modern drugs and almost one-quarter of the top 200 best-selling drugs are directed against or modulate GPCR targets (approximately 30 in total). However, due to their architecture of 7 membrane-spanning helices and their strong tendency to aggregate, it's a very challenging target class.
GPCRs can be grouped on the basis of sequence homology into several distinct families. Although all GPCRs have a similar architecture of seven membrane-spanning α-helices, the different families within this receptor class show no sequence homology to one another, thus suggesting that the similarity of their transmembrane domain structure might define common functional requirements. Depending on the size of the extracellular domain three families are discriminated (FIG. 1).                Members of Family 1 (also called family A or rhodopsin-like family; panel a of FIG. 1) only have small extracellular loops and the interaction of the ligands occurs with residues within the transmembrane cleft. This is by far the largest group (>90% of the GPCRs) and contains receptors for odorants, small molecules such as catecholamines and amines, (neuro)peptides and glycoprotein hormones. Rhodopsin, which belongs to this family, is the only GPCR for which the structure has been solved.        Family 2 or family B GPCRs (FIG. 1, panel b) are characterized by a relatively long amino terminal extracellular domain involved in ligand-binding. Little is known about the orientation of the transmembrane domains, but it is probably quite different from that of rhodopsin. Ligands for these GPCRs are hormones, such as glucagon, gonadotropin-releasing hormone and parathyroid hormone.        Family 3 members (FIG. 1, panel c) also have a large extracellular domain, which functions like a “Venus fly trap” since it can open and close with the agonist bound inside. Family members are the metabotropic glutamate, the Ca2+-sensing and the γ-aminobutyric acid (GABA)B receptors.        